1. Field of the Inventions
The present inventions generally relate to thin film solar cell fabrication, more particularly, to techniques for interconnecting solar cells based on Group IBIIIAVIA thin film semiconductors.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. It should be noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. A photovoltaic cell 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. An absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a contact layer 13, which is previously deposited on the substrate 11 and which acts as the electrical contact to the absorber film 12 of the device. The substrate 11 and the contact layer 13 form a base 20 on which the absorber film 12 is formed. Various conductive layers comprising Mo, Ta, W, Ti, and their nitrides have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use the contact layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO, CdS/ZnO or CdS/ZnO/ITO stack is formed on the absorber film 12. Radiation 15 enters the device through the transparent layer 14. A metallic grid pattern or finger pattern (not shown) comprising busbars and fingers may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side.
If the substrate 11 of the CIGS(S) type cell shown in FIG. 1 is a metallic foil, a positive voltage develops on the substrate 11 with respect to the transparent layer 14 under illumination. In other words, an electrical wire (not shown) that may be attached to the substrate 11 would constitute the (+) terminal of the solar cell 10 and a lead (not shown) that may be connected to the transparent layer 14 (or to a bulbar of the metallic grid pattern that may be deposited on the transparent layer 14) would constitute the (−) terminal of the solar cell.
After fabrication, individual solar cells are typically assembled into solar cell strings and circuits by interconnecting them (usually in series) electrically, i.e. by connecting the (+) terminal of one cell to the (−) terminal of a neighboring cell. This way the total voltage of the solar cell circuit is increased. The solar cell circuit is then laminated into a protective package to form a photovoltaic module.
For a device structure of FIG. 1, if the substrate 11 is a conductive metallic foil, series interconnection of cells may be carried out by connecting the substrate 11 at the back or un-illuminated side of one particular cell to the busbar of the finger pattern (not shown) at the front or illuminated side of the adjacent cell. A common industry practice is to use conductive wires, preferably in the form of strips of flat conductors or ribbons to interconnect a plurality of solar cells to form first a circuit and then a module as described before. Such ribbons are typically made of copper, coated with tin and/or silver. For standard crystalline Si-based technology, ribbons are attached to the front and back sides of the cells in the module structure using a suitable soldering material since both the top grid pattern of the cell and the bottom contact of the cell comprise easily solderable metallic materials such as silver. High temperature solders with processing temperatures in excess of 200° C., typically in excess of 300° C., may be used in the interconnection of Si cells to form “strings” which may then be interconnected by a process called “bussing” to form the module circuit.
Unlike Si solar cells, the thin film Group IBIIIAVIA compound solar cell of FIG. 1 may be fabricated on a metallic foil substrate such as a flexible stainless steel web or aluminum alloy foil. These materials may not be easily soldered, especially since the process temperature for this type of solar cell is limited to less than about 250° C., preferably less than 200° C. Therefore, conductive adhesives are usually employed to attach the Cu ribbons to the busbar of the grid pattern and the back contact or the back surface of the substrate of such solar cells during their interconnection. Although such techniques are in use in products, the contact resistance of the electrical contacts attached by conductive adhesives to metal foil based thin film solar cells still needs to be reduced. Adhesion of the contact to the back surface of the metallic foil substrates also needs improvement. A number of solar cells are connected together typically in series via a number of electrically conductive wires or ribbons to form what is commonly called a cell “string.” Each string has a voltage equal to the sum of the voltages of the individual cells in that string. There may be one or more conducting wires connecting each successive pair of cells depending on the electrical current collection pattern which in turn depends on the size and the shape of the cells. One common way to attach ribbons to cells is to use special conductive inks that when cured, i.e. when heated and maintained at appropriate curing temperature for sufficient time, form mechanically strong bonds that conduct electricity with low resistance.
The stringing step is a significant part of the total PV module fabrication. With respect to conventional stringing, there have been limited automated tools developed for stringing thin foil solar cells. These conventional tools are also inefficient, as they rely on complex handling systems for the cells and the strings since the strings are not rigid bodies. These systems also may rely on repeated detection and positioning of the cells or the strings at various process steps, again since the strings are not rigid and also because the bond between the ribbons and the cells are extremely weak before the conductive ink is cured. Additionally, transportation of the string in these systems before the ink is cured requires elaborate measures to prevent reliability problems that could be introduced as a result weak adhesion between the ribbons and the cells.
Therefore, there is a need to develop systems and methods that will achieve secure handling and transportation of the strings during string formation while fixing the relative positions of the cells in the string with respect to each other.